Add tests for ScalateTemplateEngineFeature
Improve support for Scalate template languages other than SSP

* No need to override scalateExtension anymore (in most use cases)

Breaking changes:

* Renamed scalateExtension: String to scalateExtensions: List[String]
    * It now represents a list of extensions, in preference order
* Custom layout paths must now include the extension
    * e.g. `layout(custom.jade)`

The second breaking change is because I couldn't find a way to get
Scalate's `LayoutStrategy` to tell me whether a given layout exists,
i.e. an equivalent to `ScalateSupport#findTemplate` for layouts.
Fix tests
Merge pull request #72 from cb372/feature/multiple-tmpl-langs-support

Feature/multiple tmpl langs support